1. Field of the Invention
The present invention relates to a method for crystallizing a substance from a solution by cooling.
2. Description of the Prior Art
In the production of fats, it is necessary to fractionate triglyceride mixtures and fatty acids. In the methods employed, the individual groups of components are separated from a solution by crystallization. The solvents used may be, for example, hexane, isopropanol, acetone, petroleum ethers or similar organic solvents.
The super-saturation necessary for the change of phase from liquid phase to crystallization phase may in principle be achieved by a change in the concentration, temperature or pressure of the solution, but in practice it is generally the temperature which is changed, i.e. the solution is cooled.
The heat is removed by convection in crystallizers equipped with stirrers which may in addition be equipped with special installations for classification of the crystals according to their particle sizes.
Since relatively little heat exchange takes place over the surface of such a crystallizer equipped with stirrer, it is quite customary to use crystallizers with a scratched heat exchange surface, so-called "scratched coolers", in which the heat transfer is much improved compared with that of convection coolers.
The crystallizers and scratched coolers used in particular for fat fractionating are designed to operate batchwise, with the result that only relatively long crystallization or dwell times of the order of ca. 30 minutes can be achieved. The maximum cooling rates obtainable are in the order of 80.degree. C. per hour.
A further disadvantage of the known methods is that they cannot be operated continuously, i.e. the entire crystallization takes place in a container from which the crystallizing mass must be removed. Such discontinuous operation is disadvantageous for technical reasons related to the process.
Lastly, the crystals produced by the known methods are irregular in shape so that the filtration which must subsequently be carried out often leads to technical problems.